The field of the invention relates to the transient heating and regrowth of thin layers of semiconductor material and more particularly to laser-scanning systems for surface heating and annealing, for example, of ion-implantation damage in semiconductor material.
The process of ion implantation is a technique used in the fabrication of a wide range of semiconductor devices from simple P-N junction solar cells to complicated LSI integrated circuits. During the process of implantation, numerous defects are created in the surface of the wafer necessitating a subsequent annealing process to provide the crystalline structure necessary for the proper electronic performance. This annealing has typically been performed via high temperature baking in ovens for sufficiently long time periods to allow various ions to migrate to lattice sites.
However, this thermal annealing has several drawbacks among which are: (1) the long exposure at high temperatures may result in migration of the implanted ions resulting in less than optimum junction characteristics; (2) during this long time period defects tend to be produced especially due to contamination; (3) minority carrier lifetimes are reduced in the underlying material since heating is essentially uniform throughout the wafer thickness.
Recently an alternative technique for ion-implantation annealing has been investigated. This technique utilizes high-intensity laser radiation to provide surface heating of implanted wafers thereby permitting localized annealing in very short time periods; it has been termed laser ion implantation annealing. As a result of the extremely short irradiation times (typically fractions of a microsecond), the problems of dopant migration, contamination and lifetime reduction may be greatly reduced by this technique.
Early experiments with short-pulse laser radiation demonstrated the ability to provide localized surface heating and thereby produce special effects. F. E. Harper and M. I. Cohen, Solid State Electronics 13 1103 (1970). Further efforts were limited at that time by the availability of lasers with precisely controllable, stable spatial and temporal beam distributions. In 1975, a number of Russian workers demonstrated the potential salubrious effects of laser ion implantation annealing. G. A. Kachurin, N. B. Pridachin and L. S. Smirnov, Sov. Phys., Semicond 9, 946 (1976); E. I. Sktyrkov, I. B. Khaibullin, M. M. Zaripov, M. F. Galyatudinov and R. M. Bayazitov, Sov. Phys., Semicond. 9, 1309 (1976) and references included therein; O. G. Kutukova and L. N. Streltsov, Sov. Phys., Semicond. 10, 265 (1976) as well as many other references. This work was performed primarily with radiation from pulsed ruby lasers and yielded "satisfactory electrical properties without deterioration of the properties of the underlying material". (O. G. Kutukova and L. N. Streltsov, Sov. Phys., Semicond. 10, 265 (1976).)
The process of laser ion implantation annealing is based on thermal physics: That is, the heating of a surface by absorption of electromagnetic radiation and subsequent melting and recrystallization. In order that only a thin surface layer be melted, it is essential that: (1) the absorption coefficient, .alpha. at the radiation wavelength be large so that all heating takes place near the surface and, (2) that pulses be very short so that minimal thermal diffusion takes place.
For the sake of the following analysis, it will be assumed that .alpha. is 10.sup.4 cm.sup.-1 or greater (a value achievable at appropriate wavelengths and temperatures for most semiconductor materials). (At 1400.degree. C. the absorption coefficient of silicon for 1.06 .mu.m radiation is 1.7.times.10.sup.4 cm.sup.-1, at shorter wavelengths it is substantially larger.) Under this condition, a distributed heat source has an exponential distribution with respect to depth into the wafer. ##EQU1## where H is the heat absorbed per unit volume; R is the surface reflectivity;
P is the total incident power; PA1 .omega. is the 1/e.sup.2 beam radius; PA1 I.sub.o is the effective surface power density.
If the laser beam diameter is large compared to the thermal depth, .delta.,(.delta.=[k.tau.].sup.1/2, where k is the thermal diffusivity and .tau. the pulse width) then the heat flow may be treated as a one-dimensional problem. The resulting temperature distribution can be obtained using the procedures of Carslaw and Jaeger. H. S. Carslaw and J. C. Jaeger "Conduction of Heat in Solids" Oxford University Press, 1959. ##EQU2## where K is the thermal conductivity. At the surface ##EQU3## The normalized temperature .THETA.=(KT)/I.sub.o .GAMMA.) is plotted in FIG. 7; over a large range of .alpha..delta. the function can be approximated by [1-exp(-.alpha..delta.)], so that ##EQU4## For a typical value of .alpha..delta..about.1 the temperature is approximately 0.6I.sub.o .delta./K. Using values for silicon, the calculated threshold to heat the surface to the melting point, T.sub.m, therefore is ##EQU5## where .delta.=.sqroot.kt =(0.075.times.1.35.times.10.sup.-7)1/2.apprxeq.10.sup.-4 Since the reflectivity is 30%, the required incident power density is 4.3.times.10.sup.6 watts/cm.sup.2 and the threshold energy density, E.sub.t =(I.sub.o T/(R), is 0.6 J/cm.sup.2 ; this is in close agreement with measured values of about 1 J/cm.sup.2. The difference is due to the fact that it is necessary to melt to a depth of about 1 micrometer and to supply the heat of fusion, as will be calculated next.
The temperature as a function of depth into the wafer given by equation (2) can be rewritten in terms of normalized variables as, ##EQU6## where z=.alpha.x, y=.alpha..delta.. A plot of this function is shown in FIG. 8.
At a normalized depth of unity (.alpha.x=1), the temperature is approximately 70% of the surface temperature; therefore, if the threshold energy is defined as being that required to raise the temperature at this depth to the melting point, I.sub.o must be increased by about 1.4 times.
Finally, in order to melt, it is necessary to supply the heat of fusion to the annealed volume. The required energy density, E.sub.f, for this is, ##EQU7## where .rho. is the density of the material and L is the heat of fusion.
Using values for silicon, at a depth of 1 micrometer again, it is found that, ##EQU8##
Thus, the incident energy density required for annealing E.sub.A, is of the order of 0.6.times.1.4+0.75=1.6 J/cm.sup.2.
Other semiconductor preparatory processes involving transient surface heating, such as conversion of an amorphous surface layer to poly- or single crystal layers or conversion of poly crystal to single crystal or conversion of surface poly crystal grain size may be carried out in similar fashion to that described above. In some of these processes melting may not be required.